1. Field of the Invention
The present invention relates to a method of and an apparatus for bonding a light-emitting element to a board at a predetermined position thereon.
2. Description of the Related Art
Generally, a linear array of light-emitting elements such as laser diodes (LDs), light-emitting diodes (LEDs), or the like is used as a light source for image reading and outputting (recording) applications. For example, as shown in FIG. 19 of the accompanying drawings, an LED array 1 comprises a plurality of LED chips (light-emitting elements) 3 mounted on a board 2 at equally spaced intervals and arranged in a linear pattern extending in one direction. The LED chips 3 are bonded on the board 2 by silver paste, with gold wires 4 extending from the respective LED chips 3.
The LED array 1 requires that the LED chips 3 be aligned highly accurately on the board 2 such that the LED chips 3 have respective light-emitting centers spaced at equal intervals. To meet such a requirement, it is necessary to recognize the light-emitting center of each LED chip 3 from the vertical direction in which the LED chip 3 emits light. However, since the LED chip 3 is a very small chip having dimensions of 0.3 mmxc3x970.3 mm, when the LED chip 3 is attracted by a collet, the LED chip 3 is concealed almost in its entirety by the collet. Therefore, when the LED chip 3 is attracted by the collet, the collet makes it difficult to confirm the light-emitting center of the LED chip 3 or even the outer configuration of the LED chip 3.
Inasmuch as the LED chip 3 is a very small chip, it is not easy to apply an alignment mark to the LED chip 3 and hence the LED chip 3 cannot be bonded on the board 2 according to a normal alignment process. If the LED chip 3 is bonded on the basis of the recognized outer configuration thereof, then because the light-emitting center of the LED chip 3 and the center of the outer configuration thereof generally do not agree with each other, the light-emitting centers of the LED chips 3 tend to be misaligned with each other even when the LED chips 3 are positioned accurately the basis of the recognized outer configuration thereof.
Japanese laid-open patent publication No. 7-43112 discloses a method of detecting the light-emitting spot of a light-emitting element and an apparatus for positioning such a light-emitting element. According to the disclosed method and apparatus, when a semiconductor device laser chip is attracted by an attraction nozzle and delivered to a positioning location, a current is supplied to energize the semiconductor device laser chip, and a CCD camera positioned in facing relationship to a light-emitting area of the semiconductor device laser chip detects light emitted from the semiconductor device laser chip. An image generated by the CCD camera based on the detected light is supplied to a controller, which detects the position and orientation of the semiconductor device laser chip based on the image, and controls the attraction nozzle to correct the attitude of the semiconductor device laser chip.
However, since the disclosed arrangement employs the semiconductor device laser chip as a light-emitting element, it cannot be applied to LED chips whose light-emitting areas have a complex shape. Inasmuch as the semiconductor device laser chip has a chip attraction surface and a light-emitting surface which are different from each other, the light-emitting state of the semiconductor device laser chip can easily be detected while the semiconductor device laser chip is being attracted. However, it is quite difficult to detect the light-emitting center of an LED chip because the LED chip has a chip attraction surface and a light-emitting surface which are oriented in the same direction. Another problem is that when a probe is applied to the LED chip to cause the LED chip to emit light, the probe conceals the light-emitting area of the LED chip, making it difficult to detect the light-emitting center of the LED chip.
It is a major object of the present invention to provide a method of and an apparatus for bonding a light-emitting element by reliably detecting the light-emitting center of the light-emitting element and positioning the light-emitting element highly accurately and easily on a board.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative example.